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* T h e M a r t i a n C h r o n i c l e *
* October 1990 No. 5 *
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The electronic newsletter by the International Mars Patrol (I.M.P.),
an observing program of the Association of Lunar and Planetary Observers
(A.L.P.O.)
Edited by: J.D. Beish
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Sand Ships of Mars
Among one of the most exciting moments in a Mars observer's life is to
catch a dust storm on Mars on the move. It is rare indeed when one can watch
one of these great red clouds slowly move over the Martian landscape covering
over features that were dark and well defined just hours before.
While no two dust clouds are alike they nevertheless exhibit similar
characteristics. Dust clouds are frequently confused with bright white areas,
frosts, or localized fogs, and some dense white clouds. In addition, after
identification is made, it becomes difficult to distinguish active dust
clouds from fresh surface dust deposits. Such misinterpretation can make time
studies difficult.
A careful diagnosis of observational records of Mars from 1877 through
the seventeen perihelic apparitions since 1907 show evidence that Martian dust
storms have evolved in preferred southern locations near the latitude of
maximum insolation. Maximum insolation occurs during Martian southern summer
which corresponds to the terrestrial months of August, September, and October
of perihelic apparitions (Capen and Martin, 1972). Recent studies of Martian
dust storms reveal three periods in the Martian year when dust clouds and/or
storms are most likely to occur. These periods correspond to 105 dg Ls*
(northern summer), 255 dg Ls (southern spring), and 315 dg Ls (southern
summer). [Beish and Parker, 1990]. * NOTE: Ls is the Planetocentric Longitude
of the Sun.
Since 1971, the year of the "Great Dust Storm of Mars," the ALPO Mars
Recorders have suspected that these disturbances come in pairs. We have seen
that in 1971 a major dust storm occurred on 213 dg Ls, followed by a "planet
encircling" dust storm on 260 dg Ls. Again, in 1973 a major storm began on
244 dg Ls and was followed by a "planet encircling" storm on 300 dg Ls. The
Viking Lander recorded two "planet encircling" storms on 204 and 268 dg Ls.
Although the two dust storms of 1988 did not reach the "planet encircling"
stage, similar patterns emerged; during 206 dg Ls a major storm occurred,
followed by another major storm on 313 dg Ls.
Perhaps the statistical significance of the three periods above appear to
mislead us when attempting to predict when these disturbances should occur.
However, one should be especially alert when observing Mars during these times
-- even though we have passed through the first sensitive period, be watchful
in the next few weeks, alas, a major dust storm may be lurking just around the
corner!
REPORTING MARTIAN DUST CLOUDS AND STORMS
In analyzing the IMP data, the ALPO Mars Recorders have had to exercise
considerable caution in defining dust clouds. They have applied the following
criteria:
1. The sine qua non of Martian dust clouds is movement with obscura-
tion of previously well-defined albedo features. Absence of this criterion in
the present study disqualified a candidate from inclusion under dust clouds
[Beish, et al, 1990].
2. They are bright in red light. Repeat -- bright in red light!
3. Often, when these clouds reach heights of several kilometers, they may cast
shadows -- that are observable from Earth. There are numerous reports of
anomalous transient dark surface markings appearing near dust clouds, espe-
cially when the solar phase angle was reasonably large [Slipher, 1962].
A DUST CLOUD IN OCTOBER 1990
During the first week of October 1990 the phone at ALPO's International
Mars Patrol headquarters began to ring. News of bright spots on Mars, "maybe a
dust cloud or something," scoffed the cautious observers. Their words were
restrained and with good reason -- past experience has taught them not to jump
to conclusions, especially about this Martian dust storm business. After all,
these storms are very rare and often mislead the most experienced Martian dust
storm watchers!
The genesis of the latest disturbance began on September 25, 1990. Re-
porting from Paragould, Arkansas, Kermit Rhea noted that Chryse appeared
brighter than usual from the 25th. of September through October 2nd. His
observations indicate unusual activity in the regions of Chryse and
Xanthe.
Don Parker, fearful of awaking my family, avoided calling me while he
observed Mars at 0400 UT on October 4th (310 dg Ls), suspected that the
bright streak bordering northern Aromatum Promontorium in the south of Chryse
might be the beginning of a dust storm. The following night (310.5 dg Ls)
this Mars Recorder watched with delight as the dust cloud, correctly identi-
fied by Parker the night before, had moved southwest onto Eos, partially
obscuring Aurorae Sinus with what appeared as a bright oval spot followed by a
long streak. The morning side of Mars had lost its usual effect of limb
darkening and was replaced by a bright haze extending 10 degrees into its
morning. Subsequent visual reports and photographs show that a dust cloud
extended from eastern Margaritifer Sinus (30W, 0N) to just south of Aurorae
Sinus.
October 6th. (311 dg Ls) brought more meteorological delights, white
clouds now laced much of the morning limb and portions of Chryse, even a
bright white cloud was seen on the limb just to the north of Sabaeus Sinus.
Morning hazes were even brighter, extending 15 to 20 degrees away from the
limb. The dust cloud of October 5th. had moved 575 miles further southwest,
now obscuring the southern half of Aurorae Sinus and completely covered Eos
(40W, 12S). A dull streak or "dusty trail" was observed to extend from the
southeast of Aurorae Sinus over into Ophir -- one of the brightest areas on
the planet Mars.
Observers on October 7th. (311.8 dg Ls) reported an increase in meteorol-
ogy on Mars. Clouds had formed over Deucalionis Regio, Aram, and in the crea-
tor Eros. Several discrete clouds in Chryse mixing with white morning limb
clouds and hazes. The main dust cloud had moved west-southwest anther 475
miles to 50W, 20S). A peculiar Y-shaped dust cloud hung over Aurorae Sinus
extending into Ophir as if transferring dust particles in that region from
the main disturbance.
One asks what would have crossed Ray Bradbury's mind while viewing such
splendor -- might he think it was Martians on the run in their sand ships?
At last, the next day the small Martian dust storm appeared to be over.
With perfect on October 8th. (312 dg Ls), this Mars observer saw a familiar
Martian landscape, with only a few alterations to the usually dark and well
defined Aurorae Sinus. Eos appeared to have grown a little. As expected, a
dull dusty haze covered the south polar region and the morning limb was
bright with blue and reddish hazes [Beish et al, 1984].
Oh yes, all was quite again on Barzoom, lest we not forget Edgar Rice
Burroughs.
CONCLUSION
Although the October 4th. - 7th. dust disturbance is not considered a
major storm it was a significant meteorological activity. Moving over 1,000
miles of the Martian landscape it did alter small portions of previously dark
and well defined features. A secondary dust cloud appeared in Ophir, an area
over 650 miles west of the main dust cloud, on October 7th. We can theorize
that dust clouds on Mars may erupt in more than one place if the conditions
are right. Past observations of Martian dust storms indicate that the Red
Planet was very near its peak dust storm period at 315 dg Ls. [Beish et al,
1990]. Remember, 315 dg Ls will fall on October 18, 1990!
REFERENCES
Beish, J.D., and D.C. Parker, "Meteorological Survey of Mars, 1968-1985,"
Journal of Geophysical Research, Vol. 95, B9, August 20, 1990.
Beish, J.D., Parker, D.C., and Capen, C.F., "A Major Martian Dust Storm in
1984", J.A.L.P.O., Vol. 30, Nos. 9-10, August 1984.
Capen, C.F., and L.J. Martin,"Survey of Martian Yellow Storms." Bull. Amer.
Astron. Soc. 4, 374, 1972.
Capen, C.F., "A Martian Yellow Cloud - July 1971." Icarus, 22, 345-362, 1974.
Slipher, E.C., "Mars, The Photographic Study. Northland Press, Flagstaff, AZ,
108-109, 1962.
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